The degree of axonal regeneration in the adult nervous system plays a major role in determining clinical outcomes in a range of neurologic conditions, from spinal cord injury to brain trauma to stroke to chronic progressive multiple sclerosis. In part, factors extrinsic to the neuron, such as Nogo, neurotrophins, and glial scar tissue, regulate the extent of axonal regeneration (Schnell et al., 1994; Fu and Gordon, 1997; Fawcett and Asher, 1999; Grand-Pre et al., 2000; Fournier and Strittmatter, 2001; Fournier et al., 2001). In addition, it is clear that different neurons respond in various ways to the same environment and that injury induces changes in the axonal growth capacity of an injured neuron. The “conditioning” nerve lesion studies of Woolf and colleagues (Chong et al., 1999; Neumann and Woolf, 1999) showed that peripheral axotomy, but not central axotomy, generates an enhanced axonal growth state. Presumably, this is attributable to the induction of neuronal regeneration-associated genes (RAG) by peripheral axotomy.
The injured PNS undergoes a stereotypical reaction to injury characterized by Wallerian degeneration in the distal portion of the nerve (Stoll et al., 1989) and a sprouting process at the proximal site. At the molecular level, there is evidence for a coordinated neuronal gene program involved in the repair process. Previous research has identified a few components of this molecular genetic switch to axon growth, although this is likely to be a very incomplete view (for review, see Fu and Gordon, 1997; Gillen et al., 1997). In general, RAGs are also highly expressed during nervous system development, suggesting that regeneration recapitulates development.
The majority of the identified RAGs encode proteins in one of several categories: cytoskeletal proteins, neurotransmitter metabolizing enzymes, neuropeptides, cytokines, neurotrophins, and neurotrophin receptors. In particular, the changes in cytoskeletal protein expression support the notion that developmental processes are being recruited. The general trend during both development and regeneration is to upregulate tubulin (Moskowitz and Oblinger, 1995) and downregulate neurofilament proteins (Muma et al., 1990; Troy et al., 1990; Wong and Oblinger, 1990). Because microtubules and neurofilaments are differentially regulated, classic neurotransmitter systems are downregulated after axotomy (for review, see Grafstein and McQuarrie, 1978; Gordon, 1983; Zigmond et al., 1996), whereas many neuropeptides are upregulated. Axotomy-induced neuropeptides include vasoactive intestinal peptide (Nielsch and Keen, 1989), galanin (Villar et al., 1989), and neuropeptide Y (Wakisaka et al., 1991). Neurotrophic factors and their receptors play critical roles during nervous system development, and in many cases expression is increased after nerve axotomy. Nerve growth factor (Ernfors et al., 1989), brain-derived neurotrophic factor and neurotrophin-3 (Schecterson and Bothwell, 1992; Kobayashi et al., 1996), acidic fibroblast growth factor (Elde et al., 1991), platelet-derived growth factor (Sasahara et al., 1991; Yeh et al., 1991), and neuregulin (Marchionni et al., 1993) are examples in this group.
Perhaps the prototypical example of a RAG is GAP-43. Skene and Willard (1981) originally discovered GAP-43 as a rapidly transported axonal protein that is highly induced after sciatic nerve injury. GAP-43 protein is localized primarily in the axonal growth cone and is expressed during brain development. Its induction by trauma is correlated with substantial functional recovery after axonal injury (Skene and Willard, 1981; Katz et al., 1985; Skene, 1989; Gispen et al., 1991). GAP-43 plus CAP-23 overexpression supports a degree of CNS axon regeneration (Bomze et al., 2001). Although GAP-43 was first identified in a two-dimensional protein electrophoresis analysis of sciatic nerve injury (Skene and Willard, 1981), other RAGs have been identified using differential display analysis (Kiryu et al., 1995; Su et al., 1997) and expressed-sequence-tag approaches (Tanabe et al., 1999).
The identification of factors responsible for regeneration of functional neurons is critical to correcting nerve damage and restoring function to patients suffering from all forms of nerve damage. In particular, those factors demonstrated to play the strongest or most significant role in such regeneration are critical to the understanding of neuronal repair. Accordingly, there is a long-felt need to identify and understand the primary factors of neuron regeneration.